The present invention relates to the field of mixers for fluid products such as, for example, paints, varnishes, enamels and the like, placed inside closed containers such as, for example, cans, pots or drums, with particular reference to a device for controlling the clamping of such containers on mixers of the type indicated above.
Mixers of known type for fluid products generally comprise assemblies for supporting and clamping containers, provided with a pair of plates disposed opposite each other, each or both of which plates are selectively movable towards and away from each other by means of a mechanical transmission operated by an electric motor. The specific technical characteristics of a known support assembly vary according to the type and model of mixer on which the aforesaid support assembly is mounted, and in particular according to the mixing motion imparted, in use, to the container, whether it be a gyroscopic, rotatory, shaking or vibratory motion, or a combination of rotatory and vibratory motion.
Specific types of support assemblies for mixers for fluid products are illustrated in documents EP-0617998, EP-0680778 and EP-0706820 of the same Applicant.
In methods for mixing fluid products, the containers are first clamped between the pair of plates and then subjected to a motion of the type indicated above, such that the various fluid product components are thoroughly amalgamated with one another. Because the containers filled with fluid product may be of considerable dimensions and weight, it is clear that during mixing they may exert thrusts and inertial forces on the clamping plates such as to cause them to be accidentally opened or loosened. For this reason the clamping force of the plates on the container should be sufficiently high to withstand the dynamic thrusts which are generated during the mixing phases, and at the same time it cannot be increased arbitrarily, in order to avoid the risk of crushing the containers of smaller dimensions, used in the case where small amounts of fluid products are to be mixed.
U.S. Pat. No. 5,268,620 describes a system for controlling the clamping force of the plates in a clamping assembly for a mixer for fluid products, in which the current absorbed by the electric motor which controls the rotation of screws for operating the plates between which the container of fluid products is clamped is detected and measured. The principal drawback of such a system lies in the fact that the current absorbed by the electric motor also depends on factors other than the effective clamping force exerted by the plates on the container. For example, in the case wherexe2x80x94as frequently occurs in practicexe2x80x94the screws for operating the clamping plates become dirty, a greater force is necessary to move the plates and overcome the higher friction compared with the situation in which the screws are perfectly clean and lubricated. The greater amount of current that the electric motor must therefore absorb in the first case interferes with the measurement of the force effectively exerted by the clamping plates and could lead to false indications, erroneously signalling that a clamping force sufficient for clamping the plates on the container has been obtained.
An aim of the present invention is that of providing an effective device for guaranteeing continuous measurement, or measurement at any time, during any operating state of the mixer, and therefore also during the mixing phases, proportional to the intensity of the forces applied to the container of fluid product by the support assembly.
A further aim of the invention is that of providing such a device which is simple, economic, reliable over time, and simple to manufacture, assemble and maintain.
In order to fulfil the aims indicated above, the subject of the invention is a device of the type indicated in the preamble of the present description, characterised in that it comprises sensor means mounted on the clamping assembly, the sensor means being disposed in a region of the clamping assembly subjected, in use, to an effort in a predetermined direction due to the compressive forces exerted on the container, the sensors being further adapted to emit a continuous signal proportional to said effort.
As will become clearer hereinafter, one of the principal advantages of the present invention lies in the continuous measurement of the signal emitted by the sensor means which allows repeatable control of said measurement such as to detect, and above all correct, any errors of measurement.
A further advantage of the present invention lies in the continuous measurement of the stress forces exerted by the support assembly for monitoring the correct operation of the device also during the mixing phases.